Recently, from the viewpoint of conservation of the global environment, exhaust emission standards for vehicles have been regulated strictly, and a reduction in weight of automobile body is promoted for improving fuel consumption. Accordingly, automobile members are also strongly required to be reduced in weight. Among automobile members, components with complicated shapes, such as an air bag component, have a problem that a difficulty in forming is steeply increased by using a highly strengthened steel sheet (steel sheet) for reducing the weights of the components.
Because of the problem, the steel sheet used as a material of the air bag component has a tensile strength of about 540 MPa even in the highest, unlike in other automobile components.
Recently, die quenching in which hardening is carried out while forming is in practical use as means for highly strengthening automobile members. However, since the air bag component has a very complicated shape, in general, the final shape cannot be formed by one-step forming and is, therefore, formed through a plurality of steps. Accordingly, it is difficult to apply the die quenching, which forms a final shape by one-step forming, to production of the air bag component. In addition, the air bag component is required to retain excellent low temperature toughness, but, the die quenching can not provide sufficient toughness by itself.
However, the automobile members are highly required to be reduced in weight, and, at the same time, automobile members such as an air bag component are desired to be highly strengthened. Accordingly, it has been recently tried to highly strengthen and also toughen the automobile members such as the air bag component by treating the members with heat by, for example, hardening after the formation of the shapes of the members. Therefore, a thin steel sheet that is used as a material for the automobile members such as the air bag component is required to have excellent strength and toughness after heat treatment, which is applied to the members after the formation of the shapes of the members.
To such requirements, for example, Japanese Unexamined Patent Application Publication No. 2002-309344 discloses a method for manufacturing a thin steel sheet of which the average grain size of BN as precipitate in steel being 0.1 μm or more and the prior austenite grain size after the hardening being 2 to 25 μm by hot rolling a steel containing 0.10 to 0.37% of C and appropriate amounts of Si, Mn, P, S, and Al and further B and N so as to satisfy a (14B/10.8N) of 0.50 or more at a coiling temperature of 720° C. or less. It is said that the thin steel sheet produced by the method disclosed in Japanese Unexamined Patent Application Publication No. 2002-309344 can have excellent properties in hardening at low temperature for a short period of time after forming and excellent toughness after the hardening and also is low in variation of properties depending on hardening conditions.
Japanese Unexamined Patent Application Publication No. 2002-309345 discloses a method for manufacturing a thin steel sheet with impact toughness after hardening of which the average grain size of TiN as precipitate in steel being 0.06 to 0.30 μm and the prior austenite grain size after the hardening being 2 to 25 μm by hot rolling a steal containing 0.10 to 0.37% of C and appropriate amounts of Si, Mn, P, S, Al, and Ti and further B and N so as to satisfy an effective B amount of 0.0005% or more at a coiling temperature of 720° C. or less. It is said that the thin steel sheet manufactured by the method disclosed in Japanese Unexamined Patent Application Publication No. 2002-309345 can have excellent properties in hardening at low temperature for a short period of time after forming and excellent impact toughness after the hardening and also be low in variation of properties depending on hardening conditions.
Although the thin steel sheets manufactured by the methods disclosed in Japanese Unexamined Patent Application Publication Nos. 2002-309344 and 2002-309345 have excellent strength characteristics after heat treatment, the toughness after the heat treatment is insufficient and cannot satisfy the levels of recent requirement for toughness. Furthermore, the strength before the heat treatment is low, which causes a problem that the strength at the portion to which the heat treatment is not applied may be insufficient. In particular, this problem is significant when the portion without receiving heat treatment is required to have a strength of 490 MPa or more.
Accordingly, it could be helpful to solve the above-mentioned problems in conventional technology by providing a hot-rolled thin steel sheet having high strength and excellent formability, i.e., a tensile strength of 440 to 640 MPa and preferably 490 to 640 MPa and an elongation of 20% or more (gauge length GL: 50 mm) as the characteristics before forming/heat treatment that are required as an air bag component, and also having excellent strength and toughness after the heat treatment, and providing a method for manufacturing such a hot-rolled thin steel sheet.
In this description, a hot-rolled thin steel sheet with “excellent strength and toughness after heat treatment” means a hot-rolled thin steel sheet having high strength showing a tensile strength of 980 MPa or more and high ductility showing an elongation of 15% or more (GL: 50 mm) after usual water hardening and tempering treatment (water hardening at about 950° C. and tempering at from room temperature to 200° C.) and having high toughness showing a ductility-brittle fracture transition temperature vTrs of −100° C. or less in a Charpy impact test. Since the hot-rolled thin steel sheet is mainly used in functional or driving components of automobiles, the thickness thereof is less than 6 mm.